The MP3 format is the most well known of all the digital audio formats. You can find MP3s all over the Web and they work with just about everything--from your Mac or PC, to your iPod or mobile phone. They're also easy to spot with their .MP3 file extension.
Most online music stores offer songs in the MP3 format, including Amazon, eMusic, Rhapsody, and Walmart. But the most popular online music store by far, iTunes, doesn't sell MP3s at all.
MP3s get a lot of headlines, but chances are you have more AAC files than you may realize. The prevalence of AAC comes down to one major factor: Apple's iTunes software. All iTunes song purchases come in the AAC format, and unless you've tweaked some settings in the software, all the music you've ripped to iTunes from CD is probably in AAC, as well. To determine if a song is AAC, see if the file ends in the .M4A extension.
Advanced Audio Coding (AAC) files have a few technical advantages over MP3, most notably their superior efficiency at encoding music compared with MP3 files at similar bit rates. Still, AAC files aren't as universal as MP3, and would likely be a bit player if not for the overwhelming popularity of Apple's AAC-friendly iPod hardware and iTunes software.
WMA files (.WMA) are a common sight for Windows users. This codec was developed by Microsoft as part of its Windows Media suite, and the acronym stands for Windows Media Audio.
WMA had a brief heyday as a music download format around 2005-2006, but steadily fell out of fashion as music download stores transitioned to the more universal MP3 format. The continued use of WMA is attributable to the fact that it is still used as the default format for ripping CDs using Windows Media Player.
You'll also see WMA used for subscription music services such as Napster and Rhapsody, since the format lends itself well to copy protection (aka PlaysForSure).
WMA files are comparable to AAC files when it comes sound quality and file size, but in general you'll find WMA supported on a wider range of products than AAC. Of course, the one place you won't find WMA supported is the Apple iPod, which is a considerable drawback of the format considering the iPod's popularity.
WAV and AIFF files end appropriately in .WAV and .AIFF (or just .AIF) extensions and offer completely uncompressed audio. This can be a good thing if you're looking for sound that is literally identical to CD quality, but the file sizes are obnoxiously large and in most cases people can't hear the difference compared with the same music encoded as a high-bit-rate MP3.
For the sound of CD quality in a much smaller file size there's Apple Lossless, which iTunes users can select from their import settings as the desired format for ripping CDs. These files offer all the fidelity of uncompressed audio (CD, WAV, AIFF) at half the file size, and can be played on the majority of iPod MP3 players. Like AAC, Apple Lossless files end in the .M4A extension, but the audio quality is technically far superior. Because this is a proprietary format, however, the support for Apple Lossless playback is limited mostly to Apple products and software.
If you love a good story, you've likely accumulated a few Audible audiobook files. These files typically end with a .AA extension, and are used exclusively for audiobook content downloaded from Audible.com. This is a copy-protected format, so your ability to play Audible files will depend on what device or software you're using. iPod playback is supported, as well as many other portable audio players.
Last, but not least, there's FLAC, also known as the Free Lossless Audio Codec. Just like Apple Lossless, FLAC files are able to compress raw audio data (CD, WAV, AIFF) in a way that causes no degradation in sound quality. The big appeal of FLAC compared with Apple Lossless or Microsoft's WMA Lossless, is the format's nonproprietary, open-source implementation. FLAC files are not compatible with iTunes or the iPod, but more and more manufacturers are supporting the codec on portable devices.